Bump flake8 from 3.8.3 to 3.8.4

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.8.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.8.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

@@ -3,5 +3,5 @@ attrs==20.2.0
 pytest==6.1.0
 pytest-cov==2.10.1
 mock==2.0.0
-flake8==3.8.3
+flake8==3.8.4
 pyflakes==2.2.0